Exposure to light is the key mechanism that enables a proper synchronization of the body clock with the solar day cycle. Timing, duration, intensity and spectral composition of light exposure all have impact on the so-called entrainment of a person to a 24-hour circadian rhythm. In particular, light exposure on the eyes strongly controls the rest-activity pattern of a person. For certain people who have a phase shift of their internal body clock relative to the social schedules around them, exposure to bright light at well-defined times can be used to shift their body clock forward or backwards to better align it with their social needs. Also for the treatment of seasonal affective disorder, timed and regular exposure to bright light is an effective means. For biological, non-visual effects of light, especially the vertical illuminance at the eye position is of particular relevance. This vertical illuminance needs to exceed a certain threshold level for enough time a day in order to achieve a healthy, well-entrained and stable circadian rhythm.
Hence, the assessment of light levels, especially of vertical light levels, to which a person is exposed over the course of multiple days or even weeks is an important instrument for the diagnosis of human behavior and physical activity. To this end, existing actigraphic products measure activity and light exposure of a user. Most popular are wrist worn devices having an integrated light sensor, e.g. Actiwatches. However, these devices do not allow for an accurate measurement of the vertical ocular illuminance of a user, since they can measure light only in one direction, which usually does not coincide with the viewing direction of the eyes.
In present actigraphic devices, the light sensor module is usually mounted on a front surface such that the axis of main sensitivity is orthogonal to a display surface of the device, i.e. orthogonal to the outer hand surface of the user. Thus, the measurement direction depends on the position of the wrist and on the body posture, so that the axis of main sensitivity of the sensor is completely independent from the direction of gaze of the user. For instance, when people are standing with their arms pointing downwards, the device will measure vertical illuminance. However, when people are seated with their arms on the lap or desk, the device will measure light exposure in a rather horizontal orientation. Since for biological, non-visual effects of light it is important to evaluate the light exposure at the eye in the vertical direction, the current wrist worn devices do not give an optimal measure of the vertical ocular illuminance. It is estimated that their light measurements reflect less than 0.5 of the actual light exposure at eye level. Moreover, they do not take account for situations, such as sensor coverage or the like, where measurements of illuminance might not be possible due to coverage of the sensor by clothing (long sleeves).
Such a wrist-worn device is shown, for example, in US 2008/0319354 A1, showing a system and method for monitoring information related to sleep. The wrist-worn device shown in this document comprises an illumination sensor to provide information related to the intensity of ambient illumination of the user. The signal of the sensor can be further processed by suitable electronic computing means.
US 2013/0100097 A1 relates to a device and method of controlling lighting of a display based on ambient lighting conditions.
KRISTOF VAN LAERHOVEN ET AL., “Sustained logging and discrimination of sleep postures with low-level, wrist-worn sensors”, WEARABLE COMPUTERS, 2008. ISWC 2008. 12th IEEE INTERNATIONAL SYMPOSIUM ON, IEEE, PISCATAWAY, N.J., USA, 28 Sep. 2008 (2008 Sep. 28), pages 69-76, relates to a study evaluating the use of simple low-power sensors for a long-term coarse grained detection of sleep postures.
WO 2011/089539 A1 relates to a control device, wearable device and lighting system for light therapy purposes.
In the following, directional illuminance is defined as luminous flux per area on a plane in this direction. For instance, vertical or horizontal illuminance refers to light falling on a vertical or horizontal surface, respectively.